The present invention relates to the technical field of the preparation of nitric esters (also referred to synonymously as nitroesters or nitrate esters) of monohydric or polyhydric (cyclo)aliphatic alcohols (i.e. in other words, the nitration of monohydric or polyhydric aliphatic or cycloaliphatic alcohols to form the corresponding nitric esters), in particular the purification of the crude nitrated nitric esters obtained after removal of the residual nitrating acid.
The present invention relates in particular to a process for removing impurities (in particular unreacted starting materials, reaction by-products, nitrating acid and reaction products thereof, e.g. nitrogen oxides or nitrous acid, etc.) obtained in the preparation of aliphatic or cycloaliphatic nitric esters, in particular a process for removing impurities from crude nitrated nitric esters obtained in the nitration of nitratable monohydric or polyhydric aliphatic or cycloaliphatic alcohols after removal of the residual nitrating acid.
Furthermore, the present invention relates to an apparatus or plant for removing impurities obtained in the preparation of aliphatic or cycloaliphatic nitric esters, in particular an apparatus or plant for removing impurities from crude nitrated nitric esters obtained in the nitration of nitratable monohydric or polyhydric aliphatic or cycloaliphatic alcohols after removal of the residual nitrating acid.
Finally, the present invention relates to a production plant for the nitration of nitratable aliphatic or cycloaliphatic alcohols with subsequent purification of the crude nitrated products (i.e. with subsequent purification of the resulting crude nitric esters) or a production plant for the preparation of nitric esters of monohydric or polyhydric aliphatic or cycloaliphatic alcohols with subsequent purification of the crude nitrated products.
Aliphatic nitroesters (also referred to synonymously as aliphatic nitric esters, aliphatic nitrate esters or the like), e.g. pentaerythrityl tetranitrate (PETN), glyceryl trinitrate (NGL), ethylene glycol dinitrate (EGDN), ethylhexyl nitrate (EHN), etc., are generally prepared in a continuously operated plant (e.g. in the case of NGL, EGDN or mixtures thereof by the process of Schmid-Meissner or Biazzi, by the injector process or by the N.A.B. process of Nielsen and Brunnberg or in the case of EHN by a modified process as described in EP 1 792 891 A1) by reaction of the corresponding alcohol (e.g. pentaerythritol, glycerol, ethylene glycol, ethylhexylethanol, etc.) with nitric acid either directly and alone or else in the presence of sulfuric acid as catalyst and water-binding agent. The nitration of the alcohols (i.e. the reaction thereof to form nitric esters) is generally carried out in the liquid phase, either as pure substance or else as a mixture or else in solution in inert, water-immiscible solvents.
The resulting crude nitroesters (hereinafter also referred to synonymously as crude nitric esters, crude nitrate esters, crude nitrated products or the like) have to be subjected before further processing thereof to multistage washing and additional purification steps in order to remove impurities which are still dissolved or suspended in the crude nitroesters, e.g. sulfuric acid, nitric acid, nitrous gases, etc., and by-products from the oxidative degradation of the starting alcohols, and thus improve the stability of the nitroesters, which are largely highly sensitive explosive materials.
The washing of the crude nitroesters in order to remove the dissolved and suspended acids of the nitration mixture and other acidic impurities or impurities which can be extracted in another way by the washing medium usually consists of three steps (see, for example, T. Urbanski, Vol. 2, pages 97 ff., in particular FIGS. 43, 44, 59 and 60, Pergamon Press, Reprint 1985). Water is usually employed as washing medium for this purpose. Washing is generally carried out as a liquid/liquid wash using the nitroester which is liquid at the washing temperature or as a solution of the nitroester in a suitable solvent.
This three-stage wash of the crude nitrated products (i.e. the crude nitroesters or nitric esters) usually comprises the following steps:                1. an acid wash (“acid wash”) with water to remove the dissolved and suspended mineral acids, e.g. sulfuric acid, nitric acid and nitrous gases;        2. a basic or alkali wash (“basic wash” or “alkali wash”) in the presence of a base, e.g. sodium carbonate (soda), sodium bicarbonate, ammonia, sodium hydroxide, potassium hydroxide, etc., in particular for removing the mineral acids still present in the nitrate ester after the first wash and residues of nitrous gases and other weakly acidic impurities from oxidative decomposition of the alcohol used or other aliphatic or cyclic hydrocarbons which were present as traces in the starting alcohol;        3. a wash with water (“neutral wash”) for removing the residual traces of alkali and for further in traces in the product.        
However, washing of the crude nitrate esters directly with soda solution, i.e. with omission of the “acid wash” stage, is sometimes also practiced and is prior art (see, for example, T. Urbanski, Vol. 2, Biazzi process, pages 107 ff., in particular FIG. 48, Pergamon Press, Reprint 1985; T. Urbanski, Vol. 4, page 328, Pergamon Press 1984; B. Brunnberg, Industrial and Laboratory Nitration, ACS Symposium Series No. 22 (Editors: L. F. Albright and H. J. T. Hanson), page 341, Washington D.C., 1976).
The objective of these washing steps is to obtain not only a pure product having high stability but also very little wastewater per metric ton of product; the washed-out impurities and the nitrate ester (traces) still present as a function of their solubility should be present in the wastewater in such amounts that they can be disposed of inexpensively.
To minimize the amount of water required for this wash and to recover the acids dissolved in the crude nitrate esters in appreciable amounts, specially nitric acid, the wash can, as described, for example, in DE-C 505 424 and DE-C 546 718, be carried out using a reduced amount of washing water in the first washing step (“acid wash”) in such a way that a washing acid having a content of nitric acid of from 20 to 60% strength or of from 35 to 55% and a content of sulfuric acid of up to 15% is obtained. This washing acid can be worked up directly or together with the residual nitrating acid.
Furthermore, washing can, for example, be carried out in countercurrent by feeding the washing water from the third washing step (“neutral wash”) either directly or after addition of bases to the washing of the nitrate ester in the second washing step (“alkali wash”) and introducing this washing water from the second washing step (i.e. the alkaline washing stage) into the first washing step (i.e. the acid wash) (cf. DE-C 505 424 and DE-B 1 135 876).
It is usual, for reasons of safety, to give preference to using injectors (jet pumps) as conveyors using the washing medium as driving medium between the individual washing steps in these three washing steps and carrying out the actual wash in air-operated washing columns. After washing is complete, the purified nitrate ester is transported as emulsion with water into a storage facility or to further processing.
DE-C 710 826 describes a process in which the three-stage washing of nitrate esters of polyhydric alcohols is carried out in countercurrent in washing columns stirred by means of air, with the nitrate esters being premixed with the washing liquid in injectors and this premixed emulsion being fed by means of air injectors into the washing column. At the end of the wash, the washed nitrate ester is, after phase separation by means of injectors which are operated using circulated driving water, transported to the storage facility. In the washing processes documented in T. Urbanski, Vol. 2, pages 97 ff., in particular FIGS. 59 and 60, Pergamon Press, Reprint 1985, too, the washing process described in DE-C 710 826 is used in combination with the injector process.
To remove a material from a mixture of materials by extraction/washing in a material which is immiscible with this mixture of materials, as in the present case of the mixture of liquid nitrate esters/impurities with water, it is necessary, in order to achieve successful extraction of the impurities from the mixture of materials, to disperse the two immiscible phases in one another, whether as oil-in-water emulsion (O/W emulsion) or as water-in-oil emulsion (W/O emulsion), in such a way that there is a sufficiently large exchange area between the immiscible phases over a sufficiently long time for the partition equilibrium for the materials to be extracted to be reached and the extracted material in the extract phase to be able to be altered by means of subsequent reactions in such a way that backextraction is no longer possible.
It is known that injectors or jet pumps are suitable for mixing miscible liquids but that injectors are unsuitable as dispersing device for dispersing two immiscible liquids in one another. The droplet sizes generated in the injector are not small enough for good washing. The stability of the emulsion produced is too low and the exchange area required for optimal mass transfer is, in combination with the short residence time in the mixture tube of the injector (about 1 second), therefore too small.
The relatively unstable nitrate ester emulsions in water which are produced using injectors therefore tend to undergo quick phase separation. This is particularly problematical for transport of such emulsions over relatively long distances. As DE-C 973 718 appears to describe, if an emulsion is conveyed alone, separation into explosive oil and water easily takes place in the long transport pipes having a small inclination. It is possible for a contiguous explosive oil layer to be formed, and this makes detonation throughout possible.
To prevent coalescence of the nitrate esters after leaving the injector and especially to prevent detonation throughout of explosive oil which has separated out during transport of the emulsion, methods which allow the emulsion of the nitrate ester in water to be stabilized and the formation of liquid threads of nitrate esters which have separated out, which are capable of detonation throughout, to be prevented have been reported, for example as described in U.S. Pat. No. 2,140,897 A, DE-C 820 575, DE-C 973 718, DE-B 1 058 093, DE-A 1 571 221 or DE-A 2 055 093. This is preferably brought about by mixing in of air or by re-emulsification using air or else by interrupting the emulsion stream with water or air.
However, it is known that gas bubbles in the liquid nitrate ester can not only significantly increase its shock sensitivity, caused by adiabatic compression (cf. DE-A 1 571 221), but that the two-phase mixtures comprising additionally drawn-in air, as previously described in DE-B 1 058 039, can lead to such stable emulsions that phase separation is possible only by use of centrifugal separators.
This risk of air inclusion in the liquid nitrate ester can be prevented by the measures described in DE-A 1 571 221 and DE-A 2 055 093. Interruption of the homogeneous emulsion column in the transport conduit by a nitrate ester-free water column effectively prevents formation of an explosive thread capable of detonation throughout by rapid coalescence of the nitrate ester emulsified in the injector.
The relatively unstable dispersions of nitrate ester in washing water or vice-versa which are produced by the injector with its relatively low dispersing power and the resulting nonoptimal exchange area for effective washing (additionally in combination with the very short residence times in the mixing region of the injector) allow only incomplete removal of the impurities in the nitrate ester to be purified. In order to achieve the desired purities, the injectors are therefore coupled with residence vessels with active dispersing.
For this reason, air-operated washing columns (DE-C 710 826) in combination with injectors as transport means or cascades of stirred vessels in combination with injectors (T. Urbanski, Vol. 4, page 328, in particular FIG. 46, Pergamon Press 1984) are usually used as washing apparatuses for washing the nitroesters to be purified in the individual washing stages. The use of the air-stirred washing columns is complicated and expensive and is no longer in accordance with present-day safety standards according to which the presence of air bubbles in the nitrate ester should be avoided (DE-A 1 571 221). Multistage cascades of stirred vessels are also complicated and expensive.
The use of injectors alone, i.e. without residence vessel, as proposed in DE-B 1 039 049 is also complicated and expensive. In order to achieve optimal purification by means of injectors without residence vessel and additional dispersing devices, each washing step has to be carried out in a plurality of stages in order to be able to obtain the desired purity and stability for further processing of the nitrate ester.
The emulsion transport of nitrate esters, as described in DE-A 1 571 221 and in DE-A 2 055 093, is relatively unsuitable as additional washing stage because it has only at most a polishing effect in order to remove traces of washing medium which have been carried as microemulsion from the preceding washing stages because of its relatively unstable emulsion with relatively large droplets produced by an injector.
Overall, the processes and plants for purifying nitrate esters which are known from the prior art either do not operate with high efficiency or else they are no longer satisfactory from the point of view of modern safety standards.